This invention relates generally to the quantitative detection of concentrations of gases, and more particularly to methods and apparatus for detecting concentrations of a gas based on its reaction with mercuric oxide.
Reduction gas detectors operate on the principle of flowing a gas stream to be analyzed through a heated bed of mercuric oxide (HgO). Gases in the stream that can be oxidized (referred to as xe2x80x9creducing gasesxe2x80x9d), react with the mercuric oxide to produce free mercury vapor as shown in the following general reaction:
X+HgOxe2x86x92XO+Hg
In this equation, X represents a reducing gas species and Hg is present as free mercury vapor. The mercury vapor produced in this reaction can be detected by its absorption of ultraviolet (UV) light within a sample cell forming a part of an ultraviolet photometer. An example of a reduction gas detector can be found in U.S. Pat. No. 4,411,867 of Ostrander, incorporated herein by reference.
Reactions with mercuric oxide are not specific to any particular gas species and a large number of reducing gases can react with mercuric oxide to produce mercury vapor. Gas measurement apparatus intended for quantitative measurements of specific gas species must therefore incorporate some process for isolating the gas species to be measured. One such apparatus is a gas chromatograph, which time-separates the gas sample into individual species. More particularly, this separation is obtained using a long tube or xe2x80x9ccolumnxe2x80x9d through which flows a gas stream. The exit gas flow from the column is connected to the reduction gas detector and an apparatus for injecting a precise volume of sample gas into the gas stream is located upstream of the column. The column itself is packed with a granular substance which has the characteristic of separating the different gases comprising the sample based on their molecular size or other chemical properties. In the case of columns containing molecular sieve materials, small molecules such as H2 will flow through the column faster than large molecules such as CO. It will therefore be appreciated that the difference in such properties cause each species or element of the sample to move through the column and into the detector at different times, and the gas species are detected as a series of Gaussian-shaped concentration xe2x80x9cpeaks.xe2x80x9d Starting from a single sample injection onto the column, each peak arrives at the detector in a characteristic time and the peak itself is essentially comprised of a single gas species. The height of each peak, or the integrated area under each peak, is representative of the concentration of the gas species.
In the prior art, reduction gas detectors have typically been operated at temperatures of 150-300xc2x0 C. in order to promote the desired reactions with mercuric oxide. The sample cell as well as the mercuric oxide bed were heated in this temperature range in order to prevent mercury from condensing on the interior surfaces of the sample cell. As is well known to those skilled in the art, mercury vapor is quite condensable and adheres to relatively cool surfaces. Mercury condensation within the sample cell can result in slow equilibration of the sample cell to changing mercury concentrations and therefore slow time response of reduction gas detectors. Additionally, ultraviolet sample cells include quartz (i.e. pure SiO2) windows which allow ultraviolet radiation to be transmitted through the cell. Mercury condensation on the quartz windows reduces the optical transmission of the cell due to absorption of the ultraviolet radiation by mercury condensation on the windows. This results in reducing signals for UV light sensors in the photometer, and correspondingly higher noise levels.
In general, gas detectors used in conjunction with gas chromatography must have relatively fast response times in order to accurately follow the concentration peaks created by the chromatography column. Additionally, typical gas chromatography flow rates are in the range of 20-60 cc/minute which are much lower than the 500-2000 cc/min flow rates associated with other gas measurement techniques (e.g. continuous analyzers). Gas chromatography detectors therefore preferably have small internal volumes in order to minimize concentration equilibration times to rapidly changing gas concentrations, and to reduce condensation of the flowing gas species as described previously.
Sample cells of the prior art, when embodied as a continuous sampling analyzer, were, of necessity, quite large in order to accommodate the large gas flows through the detectors. The large diameters of the prior art continuous sampling analyzer cells also transmitted relatively large quantities of ultraviolet radiation, which was desirable to reduce noise levels in the detector output signal. Sample cells of the prior art for chromatography detectors were smaller than those used for continuous sampling detectors but were still limited to a minimum diameter of 0.15 cm and a maximum length of 10 cm which were the dimensions that could still transmit adequate amounts of ultraviolet light through the passageway of the cell. That is, the diameter of the passageway of the cell was kept fairly large and the length of the cell was kept fairly short, so that a sufficient amount of light from the ultraviolet source could travel through the cell and still be detected by the ultraviolet (UV) sensor. This is because ultraviolet sources are non-coherent and, therefore, the amount of light impinging upon the UV detector is directly proportional to the diameter of the cell passageway and is inversely proportional to the square of the length of the cell. Hence, short, large diameter cells were the norm in the prior art.
The temperature of prior art chromatography detector cells were maintained at the same temperature as the HgO beds which, in practice, was in the range of 265-285xc2x0 C. Based on this relatively high temperature, the optical windows of the cell were constructed of relatively long quartz rods (approximately 5 cm in length) in order to isolate the hot cell from the temperature-sensitive ultraviolet lamp and light sensor. The amount of UV light transmitted through these rods is also quite dependent on temperature of the rod and, therefore, minor changes in rod temperature affect the amount of light impinging on the UV sensor. Minor variations in convective cooling of the rods of the prior art heated detector cells therefore introduced variations in light transmitted through the cell which were not due to mercury vapor concentration. The net affect of these variations was to increase drift and noise in the output of the light sensor.
It will therefore be appreciated that the performance of the prior art chromatography cell was limited by: a) the relatively large cell diameter and short length required for transmission of suitable levels of UV light; b) the relatively large condensation surface area of the cell due to its diameter and length; and c) the relatively high cell temperature which necessitated the requirement for optical windows comprised of quartz rods which added thermally-induced drift and noise to the detector output.
Since the sensitivity of mercury detection is directly proportionate to cell length, the ideal sample cell would be infinitely long and have zero diameter, zero internal volume, and zero internal surface area when one ignores other factors such as the amount of light in gas that could travel down the passage way of such an ideal sample cell. Additionally, the optical cell windows, if heated, would ideally be infinitely thin and therefore not prone to produce thermal convection errors.
A preferred embodiment of the present invention is directed to an improved photometer for detecting mercury vapor in a low flow-rate carrier gas. As such, it is well suited for gas chromatography for species that can be reduced in a heated mercuric oxide bed.
The sample cell of the improved photometer of the present invention is long and thin, as compared to sample cells of the prior art. The low internal surface area has eliminated the need to heat the cell, which permits very thin optical cell windows, which are essentially not prone to the production of thermal convention errors. The present invention stabilizes the temperature of an intense UV light source to provide sufficient, low noise UV light through the long, thin sample cell. As such, a fast, highly sensitive, and reliable photometer is provided by the method and apparatus of the present invention.
A preferred embodiment of the present invention therefore relates to detecting small concentrations of gases by measuring the spectral absorption of mercury vapor produced by those gases in a reduction process with a heated mercuric oxide bed. The apparatus includes an elongated cylindrical sample cell preferably operated at ambient temperatures and optimized to have a long passageway to increase the sensitivity of the photometer. A quartz window assembly is provided at each end of the sample cell such that ultraviolet light can be directed into a first window assembly, through the passageway of the sample cell, and out of a second window assembly to impinge upon an ultraviolet detector.
By providing a sample cell that is very long in proportion to the diameter of the passageway the need for heating the sample cell has been eliminated. Preferably, the sample cell is made from stainless steel, aluminum, or borosilicate glass. Also preferably, the ratio of the length of the sample cell to the diameter of the passageway through the sample cell is at least 100 to 1, which reduces internal surface area upon which mercury can condense and which increases the sensitivity of the cell.
The quartz window assemblies are preferably provided with individual heaters to encourage the evaporation of condensates on the windows. The ultraviolet lamp is also preferably provided with a heater, a heat sink, and closed loop control system to maintain the temperature of the lamp within precise limits. The elongated sample cell is preferably held in a V-block arrangement to provide a straight optical path through the passageway of the sample cell.
It will therefore be appreciated that a photometer of the present invention includes an elongated sample cell having a first end, a second end, and an elongated passageway extending between the first end and the second end. Preferably, a ratio of a length of the sample cell to a lateral dimension of the passageway is at least 100 to 1. Furthermore, the cell is preferably maintained at about ambient temperature. A first quartz window assembly is located at the first end of the sample cell and has a first port communicating with the passageway proximate to the first end, and a second quartz window assembly is located at the second end of the sample cell and has a second port communicating with the passageway proximate to the second end. A source of electromagnetic radiation (preferably UV radiation) is positioned to emit electromagnetic radiation through the first quartz window, the passageway, and the second quartz window, and a detector of electromagnetic radiation (preferably a UV detector) positioned to receive electromagnetic radiation emitted through the second quartz widow. Preferably, the sample cell is operated at about ambient temperature, and the volume of the sample cell is no greater than about 0.2 cc to provide fast transient response.
A method for measuring mercury vapor concentration in accordance with the present invention includes flowing a carrier gas through a mercuric oxide bed and then through a passageway of an elongated sample cell, where the sample cell has a length and the passageway has a lateral dimension such that a ratio of the length to the lateral dimension is at least 100 to 1. An ultraviolet light is directed through the cell to impinge upon a detector, and an output signal of detector is zeroed. Next, a gas sample is inserted into the flow of the carrier gas, where the gas sample comprises one or more substances that can be reacted with a mercuric oxide bed to form a mercury vapor. Finally, the output signal of the detector is analyzed.
The present invention provides a number of advantageous features over the prior art. For one, the sample cell is not heated, eliminating costly and potentially unreliable heaters and heater control systems. Furthermore, by not heating the cell, the quartz windows can be made much shorter than in the prior art, eliminating the noise component caused by small localized variations in temperature due to convention currents. Finally, the long sample cell of small diameter provides superior sensitivity and faster response time than shorter, wider cells of the prior art.
These and other advantages of the present invention will become apparent upon a reading of the following detailed descriptions and a study of the several figures of the drawings.